AGN environments at z<1.5 in the UKIDSS Ultra-Deep Survey (1104.2904v2)

Abstract: We investigate the environments of both X-ray and radio-loud AGN within the UKIDSS Ultra-deep Survey (UDS) using deep infrared selection to sample the galaxy density field in the redshift range 1.0 <= z <= 1.5. Using angular cross-correlation techniques we find that both X-ray and radio-loud AGN preferentially reside in overdense environments. We also find that both types of AGN cluster more strongly with those galaxies classified as passive' rather than those that are actively star-forming. We infer clustering scale lengths comparable to those of passive red galaxies, suggesting that typical AGN at these epochs reside in dark-matter halos of mass M >~ 10^13 M_sun. A closer look at the small-scale environments of the AGN reveals that the neighbouring galaxies of radio-loud AGN have U-B colours more skewed towards thegreen-valley' and the red sequence, whereas the neighbours of X-ray AGN show no difference to the general galaxy population. This suggests that although both AGN types live in overdense environments, the radio-loud AGN may be preferentially located in more evolved cluster cores, in a similar environment to low-powered radio AGN in the local Universe.